Method of welding or cutting material ultrasonically

ABSTRACT

The present invention relates to a method of welding or cutting material ultrasonically while avoiding hammering motion between ultrasonic anvil and ultrasonic horn, so-called cobblestoning. The material web is welded or cut generally transversely to its direction of movement, wherein at least one elongated ultrasonic horn is positioned obliquely to the direction of web movement, and at least on anvil corresponding to the ultrasonic horn is arranged on a rotatable cylinder whose rotational axis is essentially parallel with the longitudinal axis of the ultrasonic horn.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for ultrasonically weldingor cutting a moving web of material generally transversely to its thedirection of movement, comprising at least one elongated ultrasonic hornand at least one anvil corresponding to said horn, the web being movedbetween the at least one horn and the at least one corresponding anvil.The invention also relates to a method of ultrasonic welding or cuttingsuch a web.

TECHNICAL BACKGROUND

Many different types of apparatus for welding or cutting materialultrasonically are available commercially. A common feature of theseapparatus is that they include one or more ultrasonic horns which emithigh frequency sound waves, and ultrasonic anvil surfaces positionedopposite respective horns. The material is worked in the manner desired,by moving or otherwise placing said material between the ultrasonichorns and opposing anvil surfaces. It is common procedure to useultrasound when laminating and patterning thermoplastic material. Inthis regard, there are often used cylinders or drums that are providedwith a number of raised surfaces which function as anvils and which asthe cylinder/drum rotates punctiform welds, linear welds, etc., incoaction with one or more ultrasonic horns. Desired weld patterns andweld distribution are achieved by giving the raised surfaces differentpositions, sizes, shapes, etc. The raised surfaces may also be replacedwith comb-like elements, so as to obtain continuous welds. When wishingto make holes in the material or to clip or cut the material, this canbe best achieved by increasing the power of the ultrasound generatingunit. EP 0,092,866 describes apparatus for bonding together severallayers of material to form a laminate structure by means of ultrasound,wherein the ultrasonic anvil surfaces may have any one of the aforesaidforms, among other things.

TECHNICAL PROBLEMS

One problem experienced with ultrasonic welding in which severaldifferent materials are joined together with the aid of cylindersprovided with a plurality of ultrasonic anvils in accordance with theabove, resides in the occurrence of so-called cobblestoning, i.e.hammering motion between ultrasonic horn and anvil caused by variationin the abutment pressure therebetween. In addition to resulting in apoorer weld quality, cobblestoning also increases the wear on theultrasonic horn, primarily punctiform wear. EP 0,092,866 describes howat least one anvil surface can be held in firm abutment with theultrasonic horn as the cylinder rotates, by relative distribution ofsuch anvils on a rotatable cylinder. In this way, there is obtained aconstant total abutment pressure between the ultrasonic horn and theanvil surfaces carried by the cylinder. Cobblestoning is counteracted inthis way. Although the wear problem is alleviated to some extent it isnot resolved completely, because the pressure exerted per unit area onthe ultrasonic horn varies as the cylinder rotates and the anvils passthe horn. When the abutment surface between horn and anvil varies inmagnitude as the cylinder rotates, the ultrasonic energy delivered perunit area will normally also vary, resulting in welds of varyingquality. Even though attempts have been made to avoid this problem withthe aid of electronic control units, the energy delivered is still ableto increase momentarily and therewith influence the weld quality. Largevariations in power outtake will occur particularly when welding orcutting transversely to the direction of web movement, wherewith anviland horn will lie against one another during short repeated points intime along the whole of their respective lengths, i.e. with largevariations in said abutment surface. One result of this variation is thenecessity to restrict the speed of the web.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate the aforesaidproblems. The present invention provides an apparatus and a method ofwelding or cutting a moving web of material transversely to its movementdirection with the aid of ultrasonic means. According to a firstembodiment of the invention, an abutment surface of essentially constantsize is obtained during the entire welding or cutting procedure,therewith avoiding the earlier mentioned problems relating to powerouttake and punctiform wear. This is achieved by positioning anultrasonic horn obliquely to the web of material to be worked, and bypositioning a cylinder carrying ultrasonic anvil surfaces with itsrotational axis parallel with the longitudinal axis of the horn, suchthat the web will travel therebetween. The anvil surface extendshelically on the cylinder. The pitch of the helix is adapted so that theabutment surface between ultrasonic horn and said helically extendinganvil will move successively along the entire length of the horn as thecylinder rotates with a peripheral speed which is adapted so that thespeed of the anvil surface at the welding point will coincide withmovement of the web. By adapting the obliqueness of the ultrasonic hornand said cylinder to the direction of movement of the web, and byselecting a commensurate pitch with regard to the helically extendinganvil surface, the desired weld, or cut, can be made transversely to themovement direction of the web. In a second embodiment of the inventionwhich is primarily intended to enable the speed of the web to beincreased, the anvil is undoubtedly slightly helical, but constantcontact between horn and anvil is not necessary.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view from above of one embodiment of the present invention.

FIG. 2 is a side view of one embodiment of a cylinder provided withultrasonic anvils in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method is based on the basic concept of maintaining a constantpressure between horn and anvil and a contact surface therebetween ofconstant size when welding or cutting material ultrasonically. By thisis meant a constant total pressure and constant pressure per unit ofarea. The invention reduces the risk of uneven wear, uneven energyconsumption and so-called cobblestoning, i.e. a hammering motion betweenhorn and anvil. The invention also enables material to be worked morequickly, by extending the treatment time per unit of area, which alsoreduces the energy consumption per unit of time.

The invention illustrated by a preferred embodiment in the accompanyingFigures is easiest described with reference to the reference signs used.Thus, reference numeral 1 identifies a web of material in which a weldor a cut is to be made. The weld or cut is desired preferablytransversely to the movement direction of the web 1, as symbolized withthe arrow A. The FIG. 1 embodiment also includes an elongated ultrasonichorn 2, the extension of which is generally indicated by the arrow B. Acylinder 3 carrying a ultrasonic anvil 4 is placed parallel with theultrasonic horn 2. The rotational axis of the cylinder is referenced B'.The axes A and B define therebetween an angle β, this angle, naturally,being the same as the angle defined by A and B'. The angle β is chosenwithin the range 10°≦β≦80°, or within the range 100°≦β≦170°, preferablywithin the range 30°≦β≦60° or 120°≦β≦150° respectively.

FIG. 2 illustrates the cylinder 3 carrying the anvil 4. The anvil 4 hasa generally helical form in the longitudinal direction of the cylinder3, i.e. extends helically on the outer cylindrical surface of saidcylinder. Analogously, the anvil surface 4 has a generally constantpitch and describes a complete screw-turn, in other words extendshelically around the cylinder 3 through 360°. As before mentioned, theultrasonic horn 2 and the cylinder 3 are both positioned so that theirmutually longitudinal axes will define an angle β and β' respectively inrelation to the movement direction A of the web 1. When the angle β andβ' are adapted to the pitch of the helical anvil 4 and the rotationalspeed of the cylinder 3 is adapted to the speed at which the web 1moves, the peripheral speed of the cylinder 3 will be such that thespeed of the anvil 4 at the welding point will coincide essentially withthe speed of the web 1 and the ultrasonic welding process will takeplace at a point which moves successively along the full length of thehorn 2 and along the full length of the anvil 4 as the cylinder 3rotates. In other words, the point of abutment between the horn 2 andthe anvil 4 will wander successively from left to right as the cylinder3 rotates, as evident from FIG. 1. Consequently, a weld, or cut, will beformed, or made, in a direction transversely to the movement direction Aof the web 1. The anvil 4 extends helically through 360°, i.e. the anvil4 will constantly be in abutment with the horn 2 at one point or anotherthroughout one full revolution of the outer surface of the cylinder 3,therewith maintaining a contact surface of constant size between thehorn and the anvil and also a constant abutment pressure force. Thisembodiment also provides a desired spacing between transversal welds orcuts, preferably by selection of a cylinder 3 whose outer surface isadapted to this spacing. According to another embodiment of theinvention, the anvil 4 has a similar helical configuration although inthe case the helix extends only through part of one revolution of theouter surface of the cylinder. The angle β and β', however, are suchthat when a is the distance travelled by the web 1 in the movementdirection A of the web during the same time as the weld, or cut, is madeacross the web with a width b, then Tan β=b/a. Naturally, Tan β' willalso equal b/a.

It will be understood that the invention is not restricted to theaforedescribed and illustrated exemplifying embodiment thereof and thatother embodiments are conceivable within the scope of the inventiveconcept. For instance, the angle β and β' can be chosen to produce weldsor cuts which extend essentially obliquely across the web. The anvil 4on the cylinder 3 may also be given a pitch which is not constant, so asto obtain non-linear welds or cuts. It is also conceivable to use acylinder 3 that has more than one helically extending anvil 4,particularly when desiring a small spacing between the welds or cuts. Inthis way, a low cylinder-rotational speed is also obtained. Combinationsof welds, optionally of different configurations, can also be obtainedby providing the cylinder 3 with more anvil surfaces 4.

I claim:
 1. A method of ultrasonically welding or cutting a web (1)moving between at least one elongated ultrasonic horn (2) and at leastone corresponding anvil (4) generally transversely to its direction ofmovement (A), each elongated ultrasonic horn (2) being placed at anoblique angle (β) to said direction of web movement (A) and each anvil(4) being arranged on a rotatable cylinder (3) whose rotational axis(B') is generally parallel with the longitudinal axis (B) of the horn(2), characterized by adapting the rotational speed of the cylinder (3)to the speed of the web (1), so that the peripheral speed of thecylinder (3) is such that the speed of the anvil (4) at the weldingpoint coincides with the speed of the web (1).
 2. A method according toclaim 1, the at least one anvil extending helically along the cylinder(3), characterized by adapting the obliqueness of the horn (2) and theanvil-carrying cylinder (3) such as to form the angle (β) in relation tothe movement direction (A) of the web (1) to the helical pitch of theanvil (4) so that the point of abutment between the horn (2) and theanvil (4) will move successively along the length of the horn (2) andthe anvil (4) during a welding or cutting process, said welding orcutting process being effected essentially transversely to the movementdirection (A) of the web (1).
 3. A method according claim 2,characterized by controlling the distance between two mutuallysequential welds or cuts in said web (1) by selection of the radius ofthe anvil-carrying cylinder (3) and the number of anvils (4) carriedthereby.
 4. An apparatus for ultrasonically welding or cutting a movingweb (1) of material generally transversely to its direction of movement(A), comprising at least one elongated ultrasonic horn (2) and at leastone anvil (4) corresponding to said horn, the web being moved betweenthe at least one horn and the at least one corresponding anvil,characterized in that the at least one elongated ultrasonic horn (2) isplaced at an oblique angle (β) to said direction of web movement (A),and in that the corresponding at least one anvil (4) is arranged on arotatable cylinder (3) whose rotational axis (B') is generally parallelwith the longitudinal axis (B) of the horn (2).
 5. An apparatusaccording to claim 4, characterized in that the at least one anvil (4)is extended helically along the cylinder (3) with a constant pitch inthe direction of the rotational axis (B') of the cylinder.
 6. Anapparatus according to claim 4, characterized in that the at least oneanvil (4) is extended helically along the cylinder (3) with aninconstant pitch in the direction of the rotational axis (B') of thecylinder.